Numerical modelling of flow characteristics and hydrodynamic forces on a cylinder subject to oscillatory flow

    Research output: ThesisDoctoral Thesis

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    Abstract

    [Truncated abstract] The research carried out in this thesis has been concentrated on the numerical modelling of oscillatory flow around a circular cylinder. The main findings are listed as following: In Chapter 2, steady streaming induced by an oscillatory flow around a circular cylinder is investigated in the range of 2 ≤ KC ≤ 40. Six steady streaming patterns are detected corresponding to the six vortex shedding regimes in the range of KC numbers. In Chapter 3, steady streaming due to an oscillatory flow around a circular cylinder close to and sitting on a plane boundary is investigated. The parameters of flow investigated in this chapter include 2 ≤ KC ≤ 30. The structures of steady streaming are closely correlated to vortex shedding regimes. The gap to diameter ratio (e/D) has significant effect on steady streaming structure when e/D is smaller than 1.0. In Chapter 4, direct numerical simulation of sinusoidal oscillatory flow around a circular cylinder is carried out to study the Honji Instability. Numerical study is performed at a fixed KC number of 2 and a range of β numbers from 100 to 600. The position of each Honji vortex is stable in the range of 150 ≤ β ≤ 250, and interaction between Honji vortices are found in the range of 300 ≤ β ≤ 550. The mushroom spacing is only weakly dependent on β number but strongly correlates to the KC in β range of 50 to 500. An empirical relationship between KC and mushroom spacing is proposed. In Chapter 5, oscillatory flow around a circular cylinder is simulated using a two-dimensional (2D) finite element model and a three-dimensional (3D) finite element model respectively at seven Keulegan Carpenter number KC values of 1, 2, 5, 10, 17.5, 20 and 26.2. The numerical schemes used in the 2D and 3D models are identical. The purpose of this study is to investigate the feasibility of 2D models for simulating a seemingly 3D flow in terms of fundamental flow characteristics and hydrodynamic forces on the cylinder. The vortex structures predicted by the 2D model agree qualitatively with those by 3D model for the flow cases where strong correlations exist along the spanwise direction (e.g. KC = 10, 17.5 and 26.2). Three vortex shedding modes are reproduced by both 2D and 3D models at KC = 20, which is close to the critical KC between double-pair and three-pair regimes. The time histories of hydrodynamic force predicted by the two models agree with each other at KC = 20 from the mode-average point of view. ... The permeable seabed condition causes a slight increase of the inline force and has little effect on the lift force, compared with corresponding conditions in an impermeable bed. In chapter 7, hydrodynamic forces on a pipeline with uneven embedments on either side are investigated for a range of embedment depths and KC numbers. The flow structures around the pipeline are asymmetric due to the difference of seabed levels on the two sides of the pipeline. The degree of asymmetry increases with the increase of the difference of seabed levels on either side of the cylinder. Obvious difference exists between the hydrodynamic forces experienced by the pipeline in two succeeding halves of a period due to the asymmetric flow structure around the pipeline. The inline force and lift exerting on the pipeline are presented in the form of peak values and Fourier coefficients. The maximum error of the inline force and lift predicted by using sixth order Fourier series is about 4%.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2009

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